This invention relates to a drain/fill fitting of the general type utilized to fill and drain liquid tanks, such as commonly used in aircraft potable water and lavatory water supply tanks. Aircraft are exposed to frigid temperatures during high-altitude cruise. The environmental lapse rate (ELR), is the rate of decrease of temperature with altitude in the stationary atmosphere at a given time and location. The International Civil Aviation Organization defines an international standard atmosphere as having a temperature lapse rate of 6.49° C./1,000 m (3.56° F./1,000 Ft) from sea level to 11 kilometers (36,000 ft). From 11 to 20 kilometers (36,000 to 66,000 ft), the constant temperature is −56.5° C. (−69.7° F.). Thus, exterior aircraft surfaces and components at normal cruising altitudes in these ranges are subjected to temperatures far below the freezing temperature of water.
Components such as fittings and valves are provided with coaxial or surface-mounted heaters to prevent ice buildup which can cause a frozen plug to block passages in these devices. In addition, elaborate procedures are often required to ensure that these devices are completely free of water that can freeze at high altitudes.
Coaxial heaters are prone to electrical shorts that can cause complete failure of the heater. Surface heaters are not effective when drain/fill fittings are mounted against the skin of the aircraft or other metal surface due to the fact that the metal surface has sufficient mass to act as a heat sink that can remove heat added by the heater faster than it can be supplied. This is true even when there is a heated space behind the fitting in the aircraft interior. The exterior housing portion of the fitting will thus remain extremely cold and will quickly freeze any liquid remaining in the fitting after a filling or draining operation.